My first-year college prep (middle of the bell curve) chem students have been studying different types of chemical reactions. I taught them to use the activity series to predict whether or not a single replacement reaction occurs. They can do this fairly reliably on paper.
Today, I started with two partial equations on the board:
Mg + HCl →
Cu + HCl →
They were able to correctly predict that Mg (being more active than H) could displace H+ in HCl to form MgCl2 + H2. They were also able to correctly predict that Cu (being less active than H) could not displace H+ in HCl, and therefore no reaction would occur.
I sent them into the lab and gave each group two 50 ml Erlenmeyer flasks, each with about 10-15 ml of concentrated HCl in it, and I gave them a piece of magnesium metal and a piece of copper metal, with instructions to drop one of the metals into each flask and observe. Of course, as predicted, the magnesium reacted completely, bubbling and fizzing, getting hot, and making hydrogen gas; the copper did not react, and just sat in the flask of HCl.
After the experiment, several students asked me why the copper didn’t react.
In trying to help me understand this phenomenon, one of the things my wife suggested was that this may have been the first time these kids were given something in an experiment where nothing happened on purpose.
